Comparative life cycle assessment of sustainable energy carriers including production, storage, overseas transport and utilization

Countries are under increasing pressure to reduce greenhouse gas emissions as an act upon the Paris Agreement. The essential emission reductions can be achieved by environmentally friendly solutions, in particular, the introduction of low carbon or carbon-free fuels. This study presents a comparative life cycle assessment of various energy carriers namely; liquefied natural gas, methanol, dimethyl ether, liquid hydrogen and liquid ammonia that are produced from natural gas or renewables to investigate greenhouse gas emissions generated from the complete life cycle of energy carriers accounting for the leaks as well as boil-off gas occurring during storage and transportation. The entire fuel life cycle is considered consisting of production, storage, transportation via an ocean tanker to different distances, and finally utilization in an internal combustion engine of a road vehicle. The results show that using natural gas as a feedstock, total greenhouse gas emissions during production, ocean transportation (over 20,000 nmi) by a heavy fuel oil-fueled ocean tanker, and utilization in an internal combustion engine are 73.96, 95.73, 93.76, 50.83, and 100.54 g CO2 eq. MJ−1 for liquified natural gas, methanol, dimethyl ether, liquid hydrogen, and liquid ammonia, respectively. Liquid hydrogen produced from solar electrolysis is the cleanest energy carrier (42.50 g CO2 eq. MJ−1 fuel). Moreover, when liquid ammonia is produced via photovoltaic-based electrolysis (60.76 g CO2 eq. MJ−1 fuel), it becomes cleaner than liquified natural gas. Although producing methanol and dimethyl ether from biomass results in a large reduction in total greenhouse gas emissions compared to conventional methanol and dimethyl ether production, with a value of 73.96 g CO2 eq. per MJ, liquified natural gas still represents a cleaner option than methanol and dimethyl ether considering the full life cycle.